As a gallium nitride-based semiconductor transistor (hereinafter referred to as a GaN-based semiconductor transistor), a GaN-based semiconductor transistor that has an ohmic electrode composed of Ti/Al on an active layer composed of a GaN-based semiconductor is known (for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-359256). However, there has been a problem that when this GaN-based semiconductor transistor is used in high temperature conditions, electromigration, in which Al layered on Ti diffuses, occurs due to the low melting point of Ti. Thus, an ohmic electrode composed of Ti/Al has poor electrode durability, and it is difficult to ensure reliable operation.
With respect to this problem, JP-A Nos. 2006-165207 and 2007-273545, and Japanese Patent No. 4268099 describe techniques for forming an ohmic electrode composed of Ta/Al, in which Ta, which has a higher melting point than Ti, is used.